Internal or External Feedback for ZDBs

Question: What is the difference between internal feedback and external feedback?

 

Answer:

For example: the CY2308 vs. the CY2309. The CY2308 has external feedback and the CY2309 has an internal feedback. The main difference is with external feedback, the designer can control the feedback trace to delay the signal. This way, the designer can adjust the output skew with respect to the Refin. The CY2309's internal feedback is fixed. The designer does not have flexibility in controlling output skew with the input skew. The only way they can do skew control with internal feedback is to load CLKOUT.